1. Field of the Invention
The present invention relates generally to multi-component or hybrid composite structures that are made by molding uncured composite assemblies which are composed of a structural component that is embedded within a moldable component. The combination of a structural component with a moldable component allows one to take advantage of the added strength provided by the structural component while still being able to form composite structures that have relatively complex shapes. More particularly, the present invention is directed to eliminating the micro cracks that tend to form along the interfaces between the structural component and the moldable component during molding of the uncured composite assembly.
2. Description of Related Art
Composite materials typically include fibers and a resin matrix as the two principal components. Composite materials typically have a rather high strength to weight ratio. As a result, composite materials are being used in demanding environments, such as in the field of aerospace where the high strength and relatively light weight of composite parts are of particular importance.
A discontinuous fiber composite (DFC) material has been developed that can be accurately molded and machined to form a wide variety of relatively complex structures. This composite material is composed of randomly oriented segments of unidirectional tape that have been impregnated with thermosetting resin. This type of quasi-isotropic fiber material has been used to make molds and a variety of aerospace components. The material is available from Hexcel Corporation (Dublin, Calif.) under the trade name HexMC®. Examples of the types of parts that have been made using HexMC® are described in U.S. Pat. Nos. 7,510,390; 7,960,674 and published US Patent Application US2012-0040169-A1, the contents of which are hereby incorporated by reference.
The fibers used in many load-bearing composite structures or elements are unidirectional and continuous. Such unidirectional (UD) fibers are particularly useful when the load-bearing structure is relatively long with respect to the width and thickness of the structure. Wing spars, struts, links, frames, intercostals, beams, skins, panels, jet engine blades and vanes are examples of various aircraft structures that can be relatively long and which are designed to carry significant loads.
UD fibers are generally provided as a tape or layer of parallel continuous fiber that may or may not be impregnated with thermosetting resin. The tape or layer of UD fibers has a width and a thickness with the fibers extending unidirectionally in the length direction. The UD fiber layer can generally be shaped into curved structures provided that the tape is bent in the thickness direction. It is much more difficult to form curved structures in which the UD fiber layer is bent in the direction of the width of the UD layer. Procedures have been developed to allow a UD fiber layer to be bent in the width direction. Such procedures involve twisting the UD fibers prior to bending the UD layer in the width direction. Such procedures are described in published US Patent Applications US2010-0173143-A1 and US2010-0173152-A1 the contents of which is hereby incorporated by reference. These bending procedures allow one to form LID fiber layers into strong structural parts that have some curvature in the thickness and/or width directions. However, it remains difficult to form complex machinable structures using only UD fiber layers.
DFC material is entirely suitable for use in those situations where the desired composite structure has a relatively complex shape and/or requires post-curing machining. However, there are many situations where it is desirable to reinforce one or more sections of the DFC structure with continuous UD fibers. Such multi-component or hybrid composite structures are composed of DFC material, as the moldable component, and continuous UD fibers as the structural component. The UD fibers are embedded within the DFC material to provide structural reinforcement in those areas of the structure that require the extra strength which is provided by continuous UD fibers.
DFC/UD hybrid composite structures are generally made by first forming an uncured composite assembly that includes continuous UD fibers as the structural component of the assembly and DFC material as the moldable component. This assembly is cured in a mold under high pressure at an elevated temperature to produce a multi-component composite structure. The structural component can be made up of one or more UD structural elements that are placed strategically within the structure to provide the desired degree of reinforcement for the moldable component.
DFC material and continuous UD fiber layers tend to expand at different rates as the materials are heated and cured during the molding process. The rate at which these materials expand during molding is expressed as the coefficient of thermal expansion (CTE). The micro cracking that may occur along the interfaces or boundaries between the various components is a major concern when molding hybrid composite assemblies to form multi-component composite structures. Micro cracking becomes more of an issue as the difference in CTE between the various components increases. The difference in CTE between DFC material and UD fiber layers is sufficiently large that micro cracking can become a problem when these two components are combined for molding into multi-component composite structures.
It would be desirable to provide methods for making multi-component structures from DEC materials and UD fibers where micro cracking along the interfaces between the two materials is avoided during high temperature molding. Elimination of micro cracking is especially an issue in those situations where multiple UD structural elements are combined with DFC material and molded to form the hybrid structure.